In the production of carboxylic acids by hydrolysis of nitrites, it is common practice to use sulfuric acid as a catalyst. In this technology, however, as can be seen from the following reaction formula (a), the nitrile compound reacts with sulfuric: acid to give the objective carboxylic acid and the byproduct ammonium hydrogen sulfate in an equimolar ratio. EQU RCN+H.sub.2 SO.sub.4 +H.sub.2 O.fwdarw.RCONH.sub.2 .multidot.H.sub.2 SO.sub.4 RCONH.sub.2 .multidot.H.sub.2 SO.sub.4 +H.sub.2 O.fwdarw.RCOOH+NH.sub.4 HSO.sub.4 (a)
The byproduct ammonium hydrogen sulfate is discharged into the river or the like as an industrial waste. However, this effluent is detrimental to the earth's ecology and therefore presents problems. In addition, useful ammonia and sulfuric acid are not recovered but discarded, thus inflating the production cost and interfering with effective utilization of resources.
In recent years, energetic efforts have been made to develop a technology for treating processes of the byproduct ammonium hydrogen sulfate or a production process which would not give byproduct ammonium hydrogen sulfate. For example, there has been developed a process which, as illustrated in the following reaction scheme (b), comprises decomposing ammonium hydrogen sulfate thermally into nitrogen, SO.sub.2, and water, oxidizing the SO.sub.2 thus produced and recovering it in the form of sulfuric acid, and recycling the sulfuric acid as a catalyst for hydrolysis of nitrile compounds. EQU NH.sub.4 HSO.sub.4 +1/4O.sub.2 .fwdarw.1/2N.sub.2 +5/2H.sub.2 O+SO.sub.2 SO.sub.2 +1/2O.sub.2 +H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 (b)
However, in this process, too, pyrolysis of ammonium hydrogen sulfate liberates nitrogen gas and does not permit recovery of ammonia. In addition, nitrogen oxides formed on pyrolysis of ammonium hydrogen sulfate contaminate the environment. Moreover, the process from pyrolysis of ammonium hydrogen sulfate to recovery of sulfuric acid involves many complicated steps which require additional capital investment. As a result, carboxylic acids can hardly be produced efficiently and at a low cost.
Meanwhile, there has been proposed a technology for hydrolyzing a nitrile to the corresponding carboxylic acid with the aid of microorganisms. For example, Japanese Patent Publication No. 15120/1983 (JP-B-58-15120) discloses a technology for converting lactonitrile and hydroxyacetonitrile to the corresponding carboxylic acids by means of a strain of microorganism belonging to the genus Bacillus, the genus Bacteridium, the genus Micrococcus, or the genus Brevibacterium. Reported in Journal of Fermentation Technology, 51, 393 (1973) is a method for causing a yeast of the genus Torulopsis to elaborate optically active L-.alpha.-hydroxyvalerianic acid and L-.alpha.-hydroxyisocaproic acid from .alpha.-hydroxynitrile. Japanese Patent Application Laid-open No. 56086/1986 (JP-A-61-56086) discloses a technology employing a microorganism of the genus Corynebacterium to convert glyconitrile, lactonitrile and acetonecyanohydrin to the corresponding .alpha.-hydroxy acids. Japanese Patent Application Laid-open No. 84198/1990 (JP-A-2-84198) discloses a method for producing an optically active .alpha.-hydroxy acid from the corresponding .alpha.-hydroxynitrile by means of a microorganism belonging to the genus Alcaligenes, the genus Pseudomonas, the genus Rhodopseudomonas, the genus Corynebacterium, the genus Acinetobacter, the genus Bacillus, the genus Mycobacterium, the genus Rhodococcus, or the genus Candida. Japanese Patent Application Laid-open No. 40898/1992 (JP-A-4-40898 describes a method for converting .alpha.-hydroxy-4-methylthiobutyronitrile to .alpha.-hydroxy-4-methylthiobutyric acid by means of a micoorganism belonging to the genus Caseobacter, the genus Pseudomonas, the genus Alcaligenes, the genus Corynebacterium, the genus Brevibacterium, the genus Nocardia, the genus Rhodococcus, or the genus Arthrobacter.
In those processes involving microorganisms, the product carboxylic acid usually forms a salt with the byproduct ammonia. When, for the purpose of isolating the free carboxylic acid from the salt, the salt is treated with an acid such as hydrochloric acid or sulfuric acid, the ammonium salt corresponding to the acid used, such as ammonium chloride or ammonium sulfate, is formed. Thus arising are problems comparable to those mentioned in connection with the technology using sulfuric acid as a catalyst.